I. Field of the Invention
The present invention relates to an aspheric spectacle lens blank for a person following extra capsular surgery.
II. Description of the Prior Art
There are many types of previously known lenses particularly designed for aphakia patients. In one type of previously known spectacle lens, the lens comprises a curvalinear anterior surface having aspherically shaped central vision surface area and an aspherical peripheral area surrounding the central area. The aspherical surface area is formed with a variable gradient which decreases the power of the lens from the central area and to the outer periphery of the lens.
Typically, the aspherical diopter rate of change has been approximately one-third diopter per millimeter across the lens peripheral area which provides a lens power decrease of about four diopters from the center and to the periphery of the lens blank. In practice, this previously known lens improves the peripheral vision of the person suffering from aphakia when looking through the central vision portion. In order to use this previously known lens effectively, however, it was necessary for the patient to turn his or her head rather than his or her eyes in order to more clearly observe objects.
A primary disadvantage of this previously known type of lens, however, is that it is difficult for many patients to learn to focus on objects by turning their heads rather than their eyes as they have done in the past.